Input devices for visual display screens are particularly useful in cluttered computing environments where conventional data entry devices such as keyboards and mice are impractical because they occupy valuable working space.
U.S. Pat. No. 4,558,313 describes an example of a conventional electromechanical optical input device in which first and second light beams are scanned across a plane display screen by mechanical rotating beam scanners. The first light beam is scanned from one side of the screen and the second light beam is scanned from the other side. To operate the touch screen, an operator places a stylus at a desired location on the screen. The stylus interrupts portions of the light beams that would otherwise be reflected back towards their respective sources by retroreflective strips. In response to detection of the reflected portions at the sources, via photodetectors, the location of the stylus is determined from the respective angular positions of the two light beams incident on the stylus.